Generally, an image forming device such as laser printer has a developing device that supplies developer (hereinafter called “toner”) onto an electrostatic latent image formed on a photosensitive body. A conventional developing device has a toner cartridge and a developing chamber. In the toner cartridge, an agitator is provided to supply toner to the developing chamber through an opening. In the developing chamber, a toner supply roller and a developing roller are provided for supplying the toner to the photosensitive drum.
In this developing device, cylindrical toner cartridge and developing chamber are juxtaposed with each other in substantially horizontal direction and communicate with each other. If an amount of the toner in the developing chamber reaches a predetermined amount, a part of the toner flows into toner cartridge through the opening. Then, the agitator mixes the toner flowing into toner cartridge and the toner contained originally in the toner cartridge, and the mixed toner is supplied to the developing chamber. As a result, the toner circulates between the developing chamber and the toner cartridge.
In such a toner recirculation type developing device, a vertical size of the developing chamber is relatively large, so that an internal volume of the developing chamber increases to increase an amount of the toner in the developing chamber. Therefore, large amount of degraded toner remains in the developing chamber in case of replacement of old toner cartridge with a new cartridge.
In this case, the problem is that toner remaining in the developing chamber has a polarity which is opposite to an intended polarity, or toner aggregation may occur in the remaining toner. These toner may be deposited on the photosensitive drum, to degrade the quality on the image. Such disadvantageous phenomena occurs when the degraded toner and new toner is mixed with each other to some extent by the movement of the toner between toner cartridge and the developing chamber, that is, shortly after new toner is supplied to the remaining toner.